Optically variable devices are optical devices whose optical performance depends on angle of incidence of illuminating light or angle of observation. A common example of an optically variable device is an iridescent security feature used as an anti-counterfeiting measure on banknotes, credit cards, stock certificates, government-issued identification documents, etc. An optically variable device may provide a visually varying image, for example an illusory three-dimensional (3D) image, a color-shifting image, or both. Such an image is difficult to counterfeit without knowledge of a specific recipe used to manufacture the optical variable device providing the image.
Optically variable devices may be made by coating a surface with an ink or paint including flat platelet-like reflective and, or color-shifting particles. Such surfaces show higher reflectance and brighter colors than surfaces coated with a paint or ink containing conventional pigments. Substrates painted or printed with color-shifting flaked pigments may show change of color when viewed at different angles.
Flaked pigments may contain a material that is magnetically sensitive, so as to be alignable or orientable in an applied magnetic field. Such flakes may be manufactured from a combination of magnetic and non-magnetic materials and mixed with a paint or ink vehicle in the production of magnetic paints or inks. A feature of these products is the ability of the flakes to become oriented along the lines of an applied field inside of a layer of liquid paint or ink, while substantially remaining in this position after drying or curing of the paint or ink vehicle. Relative orientation of the flake and its major dimension with respect to the coated surface determines the level of reflectance or its direction and, or may determine angle-dependent color or brightness of the paint or ink.
By way of example, Phillips et al. in U.S. Pat. No. 6,808,806 disclose methods and devices for producing color-shifting images on coated articles using magnetically alignable flakes including color-shifting coatings. The color-shifting images are defined by the magnetic field applied to the coatings as the coatings are dried or cured. For example, a sheet magnet shaped as a symbol, a letter, or another indicia may be brought in close proximity to the coating during cure. After the coating is cured, the sheet magnet is removed, and the indicia may be seen as a color-shifting image on the coating. The magnetic field application may be adapted for modern printing environments; for example, Raksha et al. in US Patent Application Publication 2005/0106367 disclose a method and apparatus for orienting magnetic flakes in high-speed, linear printing operation.
A 3D illusive image may also be formed on the painted product by applying a spatially varying magnetic field to the surface of the product while the paint still is in the liquid state. When the paint is cured and the magnetic field is removed, the 3D illusive image remains visible on the surface of the painted product. The 3D illusive image appears because light rays incident on the paint layer are influenced differently by differently oriented magnetic particles. Raksha et al. in U.S. Pat. No. 7,934,451 disclose a method and apparatus to orient magnetic flakes in desired 3D patterns in a high-speed linear printing apparatus.
Despite interesting and often intriguing optical effects produced by solidified suspensions of magnetic flakes, their application in optical security devices has been somewhat limited, in particular for banknotes. The application of magnetically alignable flake suspensions in banknotes and other valuable documents may be hindered by a poor compatibility of two main printing processes mostly used in manufacturing of banknotes—offset printing and Intaglio printing—with magnetically alignable particle suspensions. An offset printing process typically produces a very thin ink film thickness, and as such, cannot transfer large magnetic particles, for example particles that are 30 micrometers in size. An Intaglio printing process typically uses a highly viscous ink, which does not allow efficient alignment of magnetic particles suspended in the ink, at least without taking special measures to lessen the viscosity of the ink while applying a magnetic field, as is disclosed by Raksha et al. in U.S. Pat. No. 8,211,509.